HK1105991B - Food can coating composition, food can coated and baked with said composition, and method for increasing the solids content of a coating formulation - Google Patents

Description

Food can coating composition, food can coated and dried therewith, and method of increasing solids content of coating formulation

This application is a divisional application of a patent application entitled "additive for low VOC aqueous coating" filed 24/10/2003 and having application number 200380100895.6(PCT/US 2003/033979).

Technical Field

The present invention relates to food can coating compositions, food cans coated and dried therewith, and methods of increasing the solids content of coating formulations.

Background

Coatings are typically applied to the interior of metal food and beverage containers to prevent the contents from contacting the metal surfaces of the containers. Contact with certain food products, particularly acidic products, can cause corrosion of metal containers. This corrosion results in contamination of the food or beverage product and deterioration of appearance and taste.

The internal protective coatings applied to metal cans are generally coatings with low extractables to avoid contamination of the contents. The coating should also be substantially defect free and should possess high resistance to various foods and beverages. Good adhesion to the metal surface and good wetting are also required to ensure complete coverage of the metal and protection of the metal during baking and forming operations. However, the high temperatures required to achieve fast cure speeds often result in blistering of the coating. Foaming typically occurs when the curing temperature exceeds the boiling point of water, which can result in incomplete or diminished coverage of the can interior.

In addition, after can manufacture, the waterborne coating must withstand the relatively stringent temperature and pressure requirements to which the can is subjected during food processing, and provide the necessary level of corrosion resistance to the can once filled.

Summary of the invention

The present invention relates to additives for low VOC aqueous coatings, including carboxylic acids and amines and alcohols. The additives may be incorporated into liquid waterborne coatings, including pigmented waterborne coatings. These coatings and methods of making these coatings are also within the scope of the present invention.

The additives of the present invention are useful for reducing the surface tension of aqueous coatings to which they are added. This results in improved "wetting" of the coating on the metal surface. "wet" is a term used in the art to refer to the ability of a coating, particularly a water-based coating, to cover a substrate with a continuous film, substantially free of defects. The use of compositions containing these additives results in very complete coverage of the metal surface of the can, especially on the inside of the can and on the already sprayed closed coating (wash coat) in the bead area of the can, which historically has been a difficult place to obtain adequate coating coverage. Better coating coverage correlates with better corrosion resistance. Importantly, the additives of the present invention enhance the performance of the coating without any negative impact on the coating. It is difficult to find additives that do not adversely affect the properties of the coating because the requirements for coating applications are very demanding; very fast high temperature curing is used in coating of the cans and the cans are subjected to very high temperatures and pressures after filling. Cans coated with a coating containing the additive of the present invention also provide excellent enamel rating (enameling) performance which ensures that the can is substantially coated. An additional advantage of the compositions of the present invention is that they have a low volatile organic solvent content (VOC). Their use is therefore environmentally beneficial.

Detailed description of the invention

The present invention relates to additives comprising a carboxylic acid, an amine and an alcohol.

The carboxylic acid can have any number of carbon atoms from about 2 to about 18 carbon atoms. It is clear that carboxylic acids having 2 to 7 carbon atoms are generally unpleasant to use and their use in certain applications may therefore be undesirable. Carboxylic acids having 8-18 carbons are therefore generally more suitable, especially for food can applications. The carboxylic acid may be obtained from or contained in an animal or vegetable fat or oil (i.e., a fatty acid). Examples of suitable acids include, but are not limited to, oleic acid, caproic acid, enanthic acid (enteronic), caprylic acid, capric acid, isodecyl acid, pelargonic acid, lauric acid, myristic acid, palmitic acid, linoleic acid, linolenic acid, stearic acid, isostearic acid, behenic acid, arachidic acid, arachidonic acid, erucic acid, azelaic acid, coconut oil, soybean oil, tall oil, tallow, lard, neatsfoot oil, almond oil, wheat germ oil, corn oil, cottonseed oil, ricinoleic acid, rapeseed oil, palm kernel fatty acids, abietic acid, dimer acid, trimer acid, ozonic acid, diacid, triacid, and mixtures thereof, as well as other acids of natural or synthetic origin. Particularly suitable are oleic acid and caprylic acid and mixtures thereof. In one embodiment, the carboxylic acid is combined with a solvent, such as pentanol and/or butanol. Carboxylic acids are commonly commercially available.

The carboxylic acids are used with amines according to the invention, such as alkylamines or alkanolamines. Examples of suitable amines include monoethanolamine, diethanolamine, triethanolamine, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, diethylethanolamine, propylamine, isopropylamine, dipropylamine, diisopropylamine, tripropylamine, triisopropylamine, or some other alkylamine or alkanolamine having from 1 to about 12 carbon atoms, or ammonia, or mixtures thereof. Especially suitable are dimethylethanolamine ("DMEA"); DMEA provides good stability and good blister resistance.

According to the invention, alcohols are also used. Any alcohol that is compatible with the carboxylic acid may be used. The alcohol may be branched or unbranched. In one embodiment, the alcohol has at least 4 carbon atoms, and in another embodiment the alcohol has 5 to 7 carbons. If the carbon number is greater than about 8, blister resistance may be compromised. Thus, one skilled in the art can select the appropriate alcohol depending on the needs of the user.

Typically, the molar ratio of carboxylic acid to amine to alcohol is from about 1.5-3.5: 11-14.5: 43-92, such as from 1.8-3.1: 12-13.5: 45-88. It should be clear that in certain embodiments of the invention, certain amines may be introduced through the epoxy acrylic resin and certain alcohols may be introduced through the phenolic resin crosslinker.

In one embodiment, the composition specifically excludes fatty acid amides.

The additives of the present invention are particularly applicable to coatings for metal food cans. The term "food can" is used herein to refer to a can, container or any type of metal receptacle for holding any type of food or beverage. Typical metals include tin-plated steel, tin-free steel, and black steel, although the invention is not limited to the use of these metals.

The invention further relates to an aqueous coating composition comprising a film-forming resin, a carboxylic acid, an amine and an alcohol. Any film-forming resin suitable for use in food cans can be used according to the present invention. Examples of polymers that can be used to form the resin include hydroxyl-or carboxylic acid-containing acrylic copolymers, hydroxyl-or carboxylic acid-containing polyester polymers, isocyanate-or hydroxyl-containing polyurethane polymers, and amine-or isocyanate-containing polyureas. These polymers are further described in US patent No.5,939,491, column 7, line 7 to column 8, line 2; this patent and the cited patents are incorporated herein by reference. Particularly suitable film-forming resins are epoxy acrylic resins such as those described in U.S. Pat. No.4,212,781, which is incorporated herein by reference. Other epoxy acrylics may also be used. Curing agents for these resins are also described in column 6, line 6 to line 62 of the' 491 patent; particularly suitable cross-linking agents, especially those for epoxy acrylic resins, include melamine and phenolic resin cross-linking agents. "phenolic resin" is understood to mean a polymer prepared by reacting one or more phenolic monomers, such as phenol, bisphenol A, tert-butylphenol, and the like, with formaldehyde. Combinations of curing agents may be used.

It should be clear that the coating of the invention is a liquid coating, more specifically a water-based or water-borne coating. For environmental reasons, water-borne coatings are preferred over solvent-borne coatings. However, it should be understood that the term "aqueous" as used herein means that the coating is primarily water; conventional solvents, such as alcohols, may be included in small amounts, such as less than 20 wt% (based on the total weight of volatiles) and remain within the scope of this invention. In fact, the introduction of small amounts of alcohol is clearly within the scope of the present invention.

Notably, the compositions of the present invention may also include pigments. Any suitable pigment may be used, including TiO₂ZnO and MgO. Pigments are added for coloring and for covering and resisting contamination of the coating of cans containing high sulfur foods, such as meat.

The compositions of the present invention may also contain any other conventional additives such as colorants, waxes, lubricants, defoamers, wetting agents, plasticizers, reinforcing agents and catalysts. Any mineral acid or sulfonic acid catalyst may be used as the catalyst. Particularly suitable for food can applications are phosphoric acid and dodecylbenzene sulfonic acid.

The film-forming resin is generally present in the aqueous coating composition in an amount of greater than about 11 wt%, such as greater than about 12 wt%, and less than 16 wt%, where the wt% is based on the total weight of the coating. When a curing agent is used, it is generally present in an amount of up to 50 weight percent; the wt% is based on total resin solids. The water or water/solvent mixture may be present in an amount of 60 to 77 weight percent, such as 62 to 74 weight percent, where the weight percent is based on the total weight of the coating. The carboxylic acid is present in the composition in an effective amount. When used with respect to a carboxylic acid, "effective amount" refers to the amount of carboxylic acid that improves the wetting ability of the coating when added to a coating. The wetting ability can be determined by means known in the art, such as the enamel rating test described herein. Clearly, various coatings known in the art may contain trace amounts of carboxylic acids (i.e., <0.1 wt%) due to incidental additions of various additives. However, such amounts are not used to improve wetting of the composition and are therefore not "effective amounts" as used herein. Typically, the wt% of carboxylic acid is 1.0 to 3.5, such as 1.5 to 2.0, where the wt% is also based on total resin solids, although even lower wt% may be used in certain formulations. The wt% of amine is typically from 17 to 23, such as from 19 to 21, where wt% is based on total organic solvent. The wt% of alcohol may be from 50 to 62, such as from 52 to 58, where wt% is also based on the total weight of the organic solvent. Pigments and other additives may comprise 11 to 15 weight percent of the composition, such as 12 and 14 weight percent, where weight percent is based on total coating weight. Typically, these compositions have 24-34% solids and 76-66% volatiles. As used herein and as understood in the art, the term "volatile" includes water.

A particularly suitable embodiment is one in which octanoic acid, alone or in combination with oleic acid, is used with an epoxy acrylic resin and a phenolic crosslinker. When oleic acid and caprylic acid are used simultaneously, the molar ratio of oleic acid to caprylic acid is generally from 1.0 to 2.5: 1.0-7.5. It has been surprisingly found that the use of octanoic acid results in a significant increase in the solids content of the coating formulation. The amount of acid used in the formulation is as described above. For example, the solids content of 22 wt% solids can be increased to 24, 25, 26, or even higher wt% solids by using octanoic acid. This high solids product (i.e., greater than 22 wt% solids) can be applied to the cans in an amount of about 250mg without foaming. This result is noteworthy. Epoxy acrylic coatings having a solids content of about 22 wt.% are generally applied to cans only in amounts of about 210-220mg and blistering still occurs. Here, it has been surprisingly found that the use of the present composition in increased amounts actually reduces, if not eliminates, foaming. Thus, the invention further relates to food can coatings having 24 wt% solids or greater comprising caprylic acid. It is also within the scope of the invention to coat and bake food cans with 250mg of the composition without significant blistering.

The carboxylic acid, amine, and alcohol can be introduced into the aqueous coating at any suitable point in the coating formation process. In a particularly suitable embodiment, the carboxylic acid and the amine are added separately during the pigment dispersion. The amine is added first, optionally followed by the defoamer, and then the carboxylic acid. The amine and carboxylic acid are not premixed, so any amine salt of the carboxylic acid that may be formed is unintentional. The addition of the carboxylic acid and amine to the pigment dispersion provides good pigment wetting and dispersion of the pigment in the coating. If different means are used to wet and disperse the pigment, the additives of the present invention may be included in the composition as post-additives. In either case, the coating has excellent wetting when applied to a metal substrate. An additional advantage of the first method is that the inventive additive facilitates the formulation of the coating by addition in a pigment dispersion. If the carboxylic acid of the present invention is not present, it is necessary to sand mill the slurry, which is more costly and time consuming than the process of the present invention.

The alcohol component of the additive of the invention may be introduced into the aqueous composition by the film-forming resin itself, if the alcohol forms part of the solvent in which the resin is contained. In addition, the alcohol may be added separately at any time during the formulation process.

The present invention therefore also relates to a process for preparing a pigmented aqueous coating composition comprising grinding a pigment with a "carboxylic acid component" comprising a carboxylic acid and an amine and mixing the ground pigment into a "resin component" comprising a film-forming resin; the carboxylic acid component or resin component further includes an alcohol. Standard methods of grinding pigments can be used. The ground pigment may then be mixed in the film-forming resin by any means known in the art; a particularly suitable process employs a cowles type mixer. Other suitable mixers include sand mills and horizontal mills.

The invention further relates to a method of preparing a pigmented aqueous coating composition comprising adding a composition comprising a carboxylic acid and an amine to a pigmented film-forming resin, wherein the resin component or the carboxylic acid component further comprises an alcohol.

The present invention further relates to a method of inhibiting corrosion on metal food cans comprising applying any of the coating compositions described above to the interior of the can. The coating may be applied by any method commonly used in the art, such as high pressure airless spray, roll coating, or coil coating. Particularly suitable is high-pressure airless spraying. The coating may be applied at a dry film thickness ("DFT") to form the desired thickness. A DFT of about 5-7 microns is generally suitable.

As noted above, any metal substrate may be used for food cans treated according to the present invention. The food can may be a two-piece or three-piece can. A "two-piece can" is understood by those skilled in the art to refer to a drawn and ironed can ("DWI"). "three-piece" food cans are understood by those skilled in the art to mean cans that are coated, formed and welded on a flat sheet. Normally, two-piece cans are coated after forming and three-piece cans are formed after coating.

The process of the invention using the above composition achieves excellent wetting on the inner surface of the can. In the preparation of DWI food cans, the coating is typically applied to the exterior of the can that is sprayed on the interior of the can; this coating is referred to as a seal coat and it typically contains a considerable amount of wax. It is noteworthy that the composition of the invention wets even on the closure coating already formed on the inside of the food can.

The ability of a coating to cover or "wet" the interior of a food can is typically determined using an enamel rating test. The test generally involves pouring brine into a tank and passing an electric current through the water; if the can is fully coated, it should have an enamel rating reading of 0 milliamps. Readings of up to about 25 milliamps are generally tolerable. The coating of the cans of the invention consistently achieves enamel rating readings below 25, such as below 4 or even below 1.

In addition to the excellent wetting properties of the coatings of the present invention, these coatings also have the additional advantage of being environmentally desirable. More specifically, these coatings have low VOC as determined by the VOC minus water calculation. The VOC of the coating of the present invention is generally less than or equal to about 3.2 pounds of solvent per gallon (minus water). VOC values of 3.0-3.2 can be consistently obtained, with VOC values as low as 2.7 or even 2.4 also being obtainable according to the present invention.

Unless otherwise specifically stated, all numbers such as those expressing values, ranges, amounts or percentages used herein are to be understood as being preceded by the word "about", even if the term does not expressly appear. Also, all numerical ranges recited herein are intended to include any sub-ranges subsumed therein. The term "polymer" as used herein refers to oligomers, homopolymers and copolymers, and the prefix "poly" refers to two or more.

Examples

The following examples are intended to illustrate the invention and should in no way be considered as limiting.

Example 1

Samples 1-5 (all of which are low VOC aqueous white interior airless spray coatings) were generally prepared as follows using the ingredients and amounts (in pounds) shown in table 1. Epoxy-acrylic dispersion (65% solids in solvent and down to about 24% solids in water) was added to a cowles tank. The mixer was turned on and dimethylethanolamine, SURFONYL104, and oleic acid (samples 1, 2, 3, 5) or 50/50 capric/butylcellosolve (sample 4) were added sequentially. Then, octanoic acid (samples 1, 4, 5), decanoic/butyl cellosolve (sample 2) or hexanoic acid (sample 3) was added; after the addition of this component, the viscosity of the mixture is significantly reduced, forming a vortex, which makes it possible to provide excellent stirring and pigment dispersion. Titanium dioxide is then added. The slurry was mixed at high speed for about 1 hour to achieve a fineness of grind ("f.o.g.") of less than 7. After a suitable pigment dispersion is obtained, melamine and deionized water are added. The above cowles dispersion was then pumped to a dilution tank (thin down tank) containing more epoxy acrylic dispersion, melamine, pentanol (samples 1-4) or 2-ethylhexanol (sample 5), carnauba emulsion, oleic acid (samples 1-3 and 5) or capric/butyl cellosolve (sample 4) and water.

The VOC of the coating was about 2.51b/gal (minus water). The coating can be sprayed under high pressure airless conditions to obtain a uniform film with excellent wetting on tin-plated steel, low enamel rating and good performance in vegetable food packaging.

TABLE 1

Components	1	2	3	4	5
						Epoxy acrylic dispersions	9.21	8.17	8.46	8.67	8.69
Dimethylethanolamine	0.16	0.14	0.15	0.15	0.15
						Surface active agent	0.42	0.37	0.38	0.40	0.39
Oleic acid	0.16	0.13	0.14	--	0.15
						Octanoic acid	0.19	--	--	0.18	0.18
Capric acid/butyl cellosolve	--	0.35	--	0.29	--
						Hexanoic acid	--	--	0.17	--	--
Titanium dioxide	13.38	11.86	13.97	12.59	12.62
						Melamine	1.16	1.01	1.06	1.09	1.09
Deionized water	4.48	4.49	5.60	5.97	5.97
						Epoxy acrylic dispersions	47.32	41.94	43.43	44.5	44.59
Melamine	5.83	5.17	5.35	5.49	5.50
						Pentanol (amyl alcohol)	1.21	1.07	1.11	0.99	--
2-Ethyl hexanol	--	--	--	--	0.99
						Brazil palm emulsion	0.19	0.18	0.19	0.19	0.19
Oleic acid	0.03	0.03	0.03	--	0.03
						Octanoic acid/butyl cellosolve	--	--	--	0.06	--
Water (W)	16.26	25.09	19.96	19.43	19.46
						Solids content(％)	32.24	27.23	30.70	29.09	29.09
Viscosity (seconds)	49	28	28	26	26
						VOC(lb/gal)	2.5	2.7	2.7	2.6	2.7

¹The dispersion may be prepared by dissolving the epoxy resin in a solventAnd then reacting the ethylene oxide with the carboxyl groups on the preformed acrylic acid using an amine as a catalyst or by dissolving the epoxy resin in a solvent and then polymerizing the acrylic monomer in the presence of the epoxy resin, which allows the epoxy resin to disperse in water once the acrylic acid is neutralized by the amine.

²2, 4, 7, 9-tetramethyl-5-decyne-4, 7-diol available as SURFYNOL104 from Air Products.

³Food grade, kosher, obtained from Acme hardest co.

⁴Obtained from Acme hardest co.

⁵50/50 capric acid butyl Cellosolve, obtained from Acme Hardesty Co.

⁶Obtained from Acme hardest co.

⁷Available as CYMEL301 from Cytec Industries.

⁸Obtained from Carroll Scientific

⁹230 ℃ F., 1 hour, as determined by ASTM.

¹⁰Measured using a #4 ford viscosity cup at 77 ° f.

¹¹ASTM solids and water were tested and determined using a VOC calculation to subtract water.

Example 2

Samples 1, 2 and 4 prepared according to example 1 were coated on 300X 407 pots to a dry weight of 300-350 mg. These cans were tin plated two-piece cans and the coating was applied by high pressure airless spray. The coated cans were subjected to an enamel rating test. The test was performed using a WACO tester from Wilkens-Anderson co, chicago, il, as indicated by the manufacturer. The solution poured into the tank, i.e. 1% electrolyteNaCl and 1% NH₄SCN. The results are shown in table 2. As can be seen in the table, all results are significantly below 25, with sample 1 averaging less than 1.3, sample 2 less than 1.0, and sample 4 less than 0.7. This indicates that excellent coverage was obtained with the composition of the present invention.

TABLE 2

Sample (I)	Enamel rating, 300X 407 cans
		1	0；0；0；0.3；1.3；4.1；0.1；6.5；3.5；0.1；0；0.5；0.9；0；1.7
2	0.1；0.5；0.8；1.6；1.0；1.4；0.6；1.3
		4	0.4；0；0；0；0.4；0.1；4.0

Example 3

The packaging test was performed using a can prepared as described in example 2 for sample 1, where the can was hot filled with food at 170 ° f and then steamed at 250 ° f for 1.5 hours. The results for the two different tanks are provided in tables 3 and 4. As seen in the table, the paint prepared in example 1 was comparable or superior to the commercial control (643E503, available from ICI) with respect to top-space adhesion and top-space corrosion of two-piece cans filled with corn steep liquor, chicken soup and pasta. Adhesion was measured by opening the can lid and cutting the can longitudinally. The tape 610 from 3M was pressed firmly against the top of the can headspace (extending downward from the top approximately 3/4 inches). The tape was then removed in one quick action and the can was visually inspected for remaining coating. "10" means that the coating was not removed. Corrosion was also determined by visual inspection; the area where the tape strips the coating is stripped is generally considered to be corroded.

TABLE 3

Paint	Packaging medium	120 DEG F storage	Adhesion of headspace	Erosion of headspace
					Sample 1	Corn steep liquor	7 days	4，4	4，6
ICI white643E503	Corn steep liquor	7 days	4，6	4，6
					Sample 1	Chicken soup noodle	7 days	4，6	4，6
ICI whi te643E503	Chicken soup noodle	7 days	2，3	2，3
					Sample 1	Pasta product	7 days	10，10	10，10
ICI white643E503	Pasta product	7 days	10，10	10，10

Injecting: the rating criteria are based on the following criteria: excellent 10 without loss

0-total loss

TABLE 4

Paint	Packaging medium	120 DEG F storage	Adhesion of headspace	Erosion of headspace
					Sample 1	Corn steep liquor	17 days	2，3	2，3

Paint	Packaging medium	120 DEG F storage	Adhesion of headspace	Erosion of headspace
					ICI white643E503	Corn steep liquor	17 days	2，4	4，6
Sample 1	Chicken soup noodle	17 days	4，4	4，4
					ICI white643E503	Chicken soup noodle	17 days	2，2	2，2
Sample 1	Pasta product	17 days	10，10	10，10
					ICI white643E503	Pasta product	17 days	10，10	10，10

Injecting: the rating criteria are based on the following criteria: excellent 10 without loss

0-total loss

Example 4

The gold color can coating according to the present invention is prepared by dissolving an epoxy resin in butanol and butyl cellosolve and polymerizing an acrylic monomer in the presence of the epoxy resin. Dimethylethanolamine was then added to the batch. The batch was then added to a dilution tank containing water and cooled from about 180F to about 150F. Then oleic acid, octanoic acid and phenolic resin were added sequentially. The batch was held at a temperature of 140-. The batch was then cooled to below 110 ° f and the wax, carnauba emulsion, was added. The viscosity was adjusted to the desired viscosity range using water in a No.4 Ford viscosity cup. The solids content of the final product was determined to be 28% (1 hour at 230 ° f according to ASTM D2360) and 25% (according to a similar procedure, but 5 minutes at 400 ° f). The viscosity of the product was 22-30 seconds, as measured using a No.4 Ford viscosity cup. The coating was applied to a 300 x 407 can in an amount of 250mg and baked in a commercial four-zone oven (where the first zone was 211 ° f, the second zone was 457 ° f, the third zone was 438 ° f and the fourth zone was 438 ° f) for a total bake time of 5-6 minutes. The coating did not blister.

TABLE 5

Components	Amount, wt%, based on total composition weight
		Epoxy acrylic resin solid	13.50
Butanol	3.5
		Butyl cellosolve	1.79
Dimethyl carbinolamine	1.35
		Water (W)	60.53
Oleic acid	0.22
		Octanoic acid	0.22
Phenolic resin	18.40
		SURFYNOL104	0.49

¹²Bisphenol a-resole resin, available as HRJ12632 from schenectady international.

While specific embodiments of the invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that various changes in detail may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims (6)

1. A food can coating composition having a solids content of greater than or equal to 26 wt% comprising octanoic acid, an amine and an alcohol, an epoxy acrylic resin, and a phenolic crosslinker, wherein the solids content is determined by ASTM D2360 at 230 ° F for 1 hour.

2. The coating composition of claim 1, wherein the solids content is 28 wt%.

3. The coating composition of claim 1, further comprising oleic acid.

4. The coating composition of claim 3, wherein the molar ratio of oleic acid to caprylic acid is 1.0-2.5: 1.0-7.5.

5. A food can coated and baked with 250mg of the coating composition of claim 1, wherein the coated food can is substantially free of blisters.

6. A method of increasing the solids content of a coating formulation comprising an epoxy acrylic resin and a phenolic crosslinker, comprising adding octanoic acid, an amine, and an alcohol to the formulation.